As it is known, in a telephone system a conventional subscriber line connecting the switching exchange (local Central Office) to the customer's equipment comprises a pair of (twisted) copper wires carrying both the telephone signals and a dc current to power the telephone electronics.
Most telephone companies in the world are bound to ensure continued telephone service to their customers for a given time duration in the event of a failure of the power distribution network (e.g. for 72 hours), and are equipped with backup generators so that under given circumstances a telephone line might be the only available source of (moderate) electrical power.
Drawing a fraction of such power for feeding the subscriber apparatus is usually allowed by the telephone companies and even the feeding of additional devices at the subscriber's premises is usually tolerated when the line is in use, i.e. in telephone jargon, when the line is in an off-hook condition.
On the other hand, the telephone companies prevent or limit the drawing of a dc power from a line in the off-hook condition for both avoiding a useless power consumption and an unauthorised feeding of other devices in the customer premises. Namely, after the subscriber has lifted the handset (or pushed a key) to be connected with the telephone exchange, a dial tone is sent to the customer apparatus and a predetermined delay is allowed for setting up the call. In case a proper selection number is not dialed and/or the call has not be set up within a given time, the line is “restrained” or set in a so called “park condition”, in which the dc power supplied through the line is reduced to a very low level and a dialing is no longer allowed. Thus the user is compelled to hang up the handset, and return to an on-hook condition. The above procedure also applies in case of a misplaced handset.
The on-hook and off-hook conditions are recognised by the central office through the current circulating in the line. Under normal (on-hook) conditions the subscriber apparatus does not draw an appreciable current (e.g. lower than 1 mA) whereas in the off-hook condition an impedance depending on the particular handset, on the line length and on other factors is connected to the line and a given range of current (loop current) is admitted.
There have been suggested devices for drawing electrical power from a telephone line when the telephone line is in the off-hook condition.
For example, US 2002/0075038 A1 discloses a power supply circuit for powering an electrical device coupled to a telephone line, such as a modem using power drawn from said telephone line when this latter is in an off-hook condition.
U.S. Pat. No. 5,461,671 discloses a telephone line power utility circuit in which dc power supplied through a telephone line in an off-hook condition is utilised as a power source for a line terminal device.
U.S. Pat. No. 5,553,138 discloses a telecommunication unit requiring power even in an on-hook condition that employs a capacitor charged in the off-hook condition. When the capacitor charge becomes too low, the unit goes off-hook for a brief period of time to recharge the capacitor.
DE 10041521 discloses a power supply procedure for extracting power from a subscriber's line during a commercial power supply failure in which a failure detection signal is output during failure of the power network at the subscriber's premises and electric power is then supplied from the station to the subscriber's line to operate the telephone circuit.
The known devices have therefore a number of limitations and drawbacks, such as:                The current drawing for feeding additional devices, when tolerated, is not under the control of the telephone company.        The amount of power that can be delivered to a device is quite limited and requires the maintaining of a telephone call, i.e. an off-hook condition of the telephone line.        There is no way to modify the power drawing limits even under circumstances that would render desirable such a power drawing.        The current drawing interferes with the normal operation of the telephone service.        
It is therefore an object of the present invention to overcome the drawbacks and limitations of the prior art devices and methods, and particularly to allow the drawing of not negligible amounts of electrical power from a telephone line for feeding electric devices.
It is another object of the invention to accomplish such current drawing in an on-hook condition of the telephone line and under the control of the telephone company running the utility.
It is a further object of the invention to realise a service available to authorised subscribers for controllably drawing a dc current from a telephone line in an on-hook condition.